It is conventional to protect a buried ferrous metal structure such as a pipeline, tank bottom or other steel or iron structure that is in contact with or partially buried in the earth by means of sacrificial anodes, as for example rods made of magnesium, that are electrically connected as by a wire to the structure to be protected. The principle of operation of such a system is that the presence of the materials, magnesium and iron, within the immediate area of each other within the soil produces an electrical couple wherein the magnesium rod becomes the anode and the steel structure becomes the cathode. Within the electrical couple, the magnesium anode selectively corrodes while a negative charge develops on the steel structure protecting it from corrosion. Corrosion is generally defined as the dissolution of the base material or metal into the surrounding environment. Iron will not generally oxidize in the presence of a negative charge or potential if that potential is sufficiently high. Reference to the electromotive chemical series shows that a potential of -0.68 volts with reference to a saturated Calomel electrode (-0.44 volts with reference to the standard hydrogen electrode), is sufficient to prevent the oxidation of iron from its elemental to an ionic form.
It has been observed that the magnesium anodes are apparently consumed by electrochemical actions other than the electrochemical reaction that protects the steel structure. This reduces the efficiency of such a magnesium anode requiring its earlier replacement. Additionally, a magnesium anode apparently loses its ability to fully protect a steel structure long before the anode is consumed; it has been observed, in many cases, that there is a gradual reduction of the electrical potential of the steel structure to less than -0.68 volts versus a saturated Calomel electrode. For a large utility company attempting to protect miles and miles of buried pipeline or other structures, the cost of replacement of magnesium anodes utilized in the cathodic protection processes is significant. When magnesium rods or anodes require replacement before they are fully consumed and are additionally consumed by processes other than those desired for the protection of the buried steel structure, waste and inefficiency occur. Significant labor and material expenditures are incurred for the untimely replacement of magnesium anodes.